Project Summary It has recently been discovered that prokaryotes can acquire resistance to viruses and plasmids by integrating short fragments of foreign DNA into clusters of regularly interspaced short palindromic repeats (CRISPR's). These repeats are then transcribed and processed into small guide RNA's that are used to direct the destruction of foreign nucleic acid. This mechanism has many parallels with eukaryotic RNA interference but the proteins that are associated with the CRISPR response are evolutionarily unrelated to their eukaryotic counterparts. Our long-term goal is to understand the biochemical and structural basis of CRISPR-mediated resistance in prokaryotes. The objective here is to determine the mechanism by which an effector protein, responsible for the ultimate destruction of the foreign invader, is recruited specifically to foreign DNA. Our objective will combine biochemical, structural and cell based experiments. Successful completion of the proposed studies is significant because it will increase our understanding of bacterial resistance to viruses and plasmids. Both of these genetic elements play important roles in the genetics of pathogenic bacteria.